Salvia Divitorium (sp?) seeds are so rare that some experts claim they don't even produce seeds. However, you can purchase live plants (from clippings) on many sites, including eBay. I'd say the best method is to go that route. As far as sites, I've seen a few through google...
Err, and as far as I know, there's not age limit in the U.S. and if there is, it's 18, which you should be

I know it's 100% legal to grow, possess, sale, etc...
Also, another cool thing you can research on is making your own salvia extract. There's some pretty shweet sites on it, even some forums.